Do not send periodic RAs on IPv6-less interfaces

Ignore interfaces with no IPv6 subnets on the respective routing tables and log
a debug message.
Signed-off-by: Apollon Oikonomopoulos <apollon@noc.grnet.gr>

nfdhcpd

@@ -695,6 +695,11 @@ class VMNetProxy(object): # pylint: disable=R0902
                 continue
 
             subnet = self.v6nets[iface]
+            if subnet.net is None:
+                logging.debug("Skipping periodic RA on interface %s,"
+                              " as it is not IPv6-connected", iface)
+                continue
+
             ifll = subnet.make_ll64(ifmac)
             resp = Ether(src=ifmac)/\
                    IPv6(src=str(ifll))/ICMPv6ND_RA(routerlifetime=14400)/\